Power-packed neutrinos as supernova detectors

Astronomers can get a more detailed understanding of very distant supernova explosions by searching for high-energy neutrinos, according to a study by researchers at Uppsala University, Sweden.

A supernova represents the cataclysmic end of a massive star’s life. As the star collapses, its iron core disintegrates, producing vast numbers of neutrinos that stream outwards in unimaginable quantities—around 10⁵⁸ (10 billion trillion trillion trillion trillion) of them. Their outward pressure blasts the star apart causing a supernova explosion.

There are various neutrino detectors across the world, including IceCube in Antarctica. They can detect the neutrinos produced by core-collapse supernovae, which typically have energies in the Million-electron Volt (MeV) range. However, the way these detectors work means that neutrinos from supernovae farther away than the Magellanic Clouds—the largest of the satellite galaxies of the Milky Way—are too faint for them to see.

Astronomers Nora Valtonen-Mattila and Erin O’Sullivan of Uppsala University believe that higher-energy neutrinos—in the 10⁹-10¹² (giga to tera) eV range—should also exist, which would extend that range to more than 10⁸ light years. That would include supernovae in many other galaxies and local galaxy clusters.

According to them, these neutrinos are not produced by the supernova itself but through two other production mechanisms. Before stars actually explode, they tend to eject vast amounts of material in fierce stellar winds. When the supernova detonates, the debris from the explosion hits this circumstellar material. Protons collide like they do in particle accelerators and should produce high-energy neutrinos in the process. However, these neutrinos have yet to be observed.

A similar thing could be achieved through a jet of material that becomes trapped behind the star’s outer shell. This is called a choked jet. Ordinary particles may be trapped, but neutrinos can stream through dense material with ease—a light year of lead would have only fifty-fifty chance of stopping a neutrino.

It is these high-energy neutrinos from choked jets that offer the greatest possible extension to IceCube’s range, pushing it out to 277 million light years in the northern sky and 65 million light years in the southern sky, according to the researchers. However, choked jets are rare, with just 1 to 4 per cent of core-collapse supernovae having one.

With the ongoing upgrade to IceCube-Gen2, which should be completed by 2033 (Frontline, April 3, 2021), these considerations should get validated giving scientists a better understanding of core-collapse supernovae and the neutrinos they produce.